Force of nature: Engineers define the mechanical mechanism in living cells

A team of researchers, led by chemical engineer Alex Dunn, measured mechanical tension at the nanoscale to explore how living cells create and detect force. The research could lead to a better understanding of how tissues and tumors form and grow, and, ultimately, of how complex living organisms organize themselves.

By Andrew Myers

If you place certain types of living cells on a microscope slide, the cells will inch across the glass, find their neighbors, and assemble themselves into a simple, if primitive tissue. A new study at Stanford University may help explain this phenomenon, and then some, about the mechanical structure and behavior of complex living organisms.

Pulling back the veil on the exact nature of this mechanism could have bearing on biological understanding ranging from how tissues and tumors form and grow, to the creation of entire complex living organisms.

There are scads of data explaining chemical signaling between cells. “And yet, one of the great roadblocks to a complete knowledge of how cells work together to form tissues, organs and, ultimately, us, is an understanding of mechanical forces,” said Dunn.

Using a new force-sensing technique, Dunn and team have been able to explore how cells connect to one another and how individual cells control their own shape and movement within larger tissues.

Seeing the force

“Cells are really just machines. Small, incredibly complex biological machines, but machines nonetheless,” said Dunn. “They rely on thousands of moving parts that give the cell shape and control of its destiny.”

The mechanical parts are proteins whose exact functions often remain a mystery, but Dunn and team have helped explain the behaviors of a few.

At its most basic level, a cell is like a balloon filled with saltwater, Dunn explained. The exterior of the cell, the balloon part, is known as the membrane. Protruding through the membrane, with portions both inside and outside the cell, are certain proteins called cadherins.

Outside of the cell, cadherins bind one cell to its neighbors like Velcro. The ‘herin’ portion of the name, in fact, shares a Latin root with “adhere.”

On the inside of the cell, cadherin is connected to long fibers of actin and myosin that stretch from membrane to nucleus to membrane again. Actin and myosin work together as the muscle of the cell, providing tension that gives the cell shape and the ability to control its own movement. Without this force, the balloon of the cell would be a shapeless, immobile blob.

Puppeteer’s string

“If you watch a cell moving across a glass slide, you can see it attach itself on one side of the cell and detach on the other, which causes a contraction that allows the cell to, bit by bit, pull itself from place to place,” said Dunn. “It’s clearly moving itself.”

While it was understood that cadherin and actin are connected to one another by other proteins known as catenins, what was not known was how, when, and where the cells might be using their muscles (actin and myosin) to tug on the Velcro (cadherin) holding them to other cells.

Dunn and his colleagues have shown for the first time that the actin-catenin-cadherin structure transmits force within the cell and, further, that cadherin can convey mechanical forces from one cell to the next. This is an important problem in the development of organisms, since a cell must somehow control its shape and its attachments to other cells as it grows, divides, and migrates from one place to another within the tissue.

Like the strings of a puppeteer controlling a marionette, this mechanism is a form of mechanical communication. Dunn and others in the field believe that these mechanical forces may be important in conveying to a cell how to position itself within a tissue, when to reproduce and when to stop as the tissue reaches its proper size and shape.

“That is the theory, but an important piece was missing,” said Dunn. “Our research shows that forces at cell-cell contacts can in fact be communicated from one cell to its neighbors. The theorized mechanical signaling mechanism is feasible.”

“The molecular basis for how cells sense and interpret mechanical forces is just coming into view,” said Douglas N. Robinson, PhD, an associate professor in the Center for Cell Dynamics at Johns Hopkins School of Medicine. “Dunn and colleagues provide evidence that E-cadherin directly experiences forces propagated between cells and, in turn, communicates forces generated internally to the plasma membrane. As such, this study identifies an important step in the force transmission within and between cells in tissues.”

Proposed model: Cadherin (orange bars) extends across the cell membrane (thick black lines), binding cells to one another. Inside the cell, cadherin is under tension generated by actin and myosin (ellipsoid spirals) connected by alpha-catenin (α) and beta-catenin (β). Illustration courtesy of Nicolas Borghi, Stanford University School of Engineering.

Story within a story

How Dunn and his colleagues got to this point is a story in itself. It reads like the recipe for a witch’s potion—cultured canine kidney cells, DNA from jellyfish and spider silk, and microscopic glass needles.

To measure the force between cells, a team combining the skill of several Stanford laboratories—headed by Professor Dunn in chemical engineering, professors William Weis and W. James Nelson in the Department of Molecular and Cellular Physiology and associate professor of mechanical engineering Beth Pruitt—used a tiny and ingenious molecular force sensor developed by Martin Schwartz and colleagues at the University of Virginia. The sensor combines fluorescent proteins from jellyfish with a springy protein from spider silk.

The genes for the sensor are incorporated into the studied cells’ DNA. Under illumination, the cells glow in varying colors depending on how much stretch the sensor is under. In this study, the force sensor is inserted into the cadherin molecules binding cells together—when the Velcro stretches, so does the sensor.

Left: Two cells bound to each other. Cadherin (white) is tagged with the molecular force sensor, which emits yellow/green fluorescent light whose color varies by the amount of mechanical stretch. Right: False-color image showing the molecular forces measured using the molecular force sensor. Blue is high force, and red is low force. Illustration courtesy of Nicolas Borghi, Stanford University School of Engineering.

The team then took things a step further. By turning the activity of myosin, actin and catenin on and off, they were able to determine that these proteins are in fact linked together and are at the heart of inter- and intra-cellular mechanical force transmission.

Lastly, using glass microneedles, the team tugged at connected pairs of cells, pulling at one cell to show that force gets communicated to the other through the cadherin interface.

“At this point we now know that a cell exerts exquisite control over the balance of its internal forces and can detect force exerted from outside by its neighbors, but we still know next to nothing about how,” said Dunn. “We are extremely curious to find out more.”

This research was made possible by funding from a Stanford Bio-X IIP award, the National Science Foundation, the National Institutes of Health and a Burroughs-Wellcome Career Award at the Scientific Interface.

Andrew Myers is associate director of communications for the Stanford University School of Engineering.